The marsupials and monotremes that are now found primarily in Australia represent a sort of evolutionary middle ground, mixing together their mammalian features with those of reptiles or amphibians. One ancient marsupial represents a particularly weird case of convergent evolution.

Malleodectes, otherwise known by the awesome nickname "the Hammer-Biter", was a marsupial that lived in Australia between 10 and 17 million years ago. Its teeth were shaped like hammers, a bizarre feature that is found in only one living creature: an Australian lizard known as a skink (pictured below). These lizards use their huge, oddly blunt teeth for one purpose, and that's what allowed the Queensland Museum researchers to figure out why these ancient marsupials had them.

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This ancient creature might have looked lizardy, or a bit like this cat-sized marsupial called a northern quoll, above. Or like some bizarre combination.

Scott Hocknull explains:

"This rainforest skink has an almost identical giant, hammer-tooth in its dentition and in this case we know what it's used for: crushing the hard shells of snails, one of the main foods of this rainforest skink.

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Fellow researcher Rick Arena adds:

"At first, the function of these teeth was a mystery because we were unaware of any other mammal that had hammer-teeth like this. It appears Malleodectes evolved millions of years ago to exploit the ecological niche occupied today by these specialised lizards."

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This phenomenon is known as evolutionary convergence, in which two completely separate species evolve the same solution to the same basic challenge. Skinks and the hammer-biters both made snails a primary part of their diet, and that was only possible if they could crush their shells to get at the meat within.

This case of convergent evolution is highly unusual. Arena says this is the first time ever that a marsupial's teeth most resemble that of a lizard, and it's generally rare for such distantly related creatures to share such similar adaptations. Coauthor Mike Archer speculates that they didn't just evolve to eat the same food source - eventually, these two species might have competed for one limited supply of snails:

"It's possible that species of Malleodectes may have survived for a bit longer in rainforest communities in eastern Australia and here found themselves in competition for snails with the similarly-specialized ancestors of the pink-tongued skinks. If this did happen, clearly, for whatever reason, these extraordinary mammals lost out to the lizards."